I. Field of the Invention
This invention relates generally to an apparatus and process for vaporizing heavy hydrocarbon feedstock. In particular, the present invention relates to an apparatus and process for substantially completely vaporizing heavy hydrocarbon feedstock mixed with steam. The invention also relates to the use of the apparatus and process in the production of gaseous and liquid products such as olefins from heavy hydrocarbon feedstock.
II. Background of the Related Art
Thermal cracking to convert hydrocarbon feedstocks into more useful gaseous and liquid by-products is a well known process. The cracking process, which is also known as pyrolysis, cracks the carbon-carbon bonds in longer hydrocarbons to form smaller chain hydrocarbons. The overall reaction rate and end products are dependent on the processing temperature, pressure and residence time. Thermal cracking has commonly been used to produce olefins such as ethylene, propylene or butene from various types of relatively light hydrocarbon feedstocks, such as ethane, propane or naphtha. Steam cracking is a high-temperature thermal cracking process, which is typically performed in a pyrolysis furnace.
A conventional pyrolysis furnace generally has two primary sections: a convection section and a radiant section. Gaseous or liquid hydrocarbon feed is introduced into the convection section where it comes into indirect contact with and is preheated by furnace burner exhaust gases. Additionally, where desired, dilution steam may be added in the convection section. The liquid hydrocarbon feed is preheated in the convection section to produce a liquid-vapor two-phase mixture. The preheated feed is then directed to a radiant section, wherein the feedstock-steam mixture is heated with radiant heat and typically cracked at temperatures as high as 850° C. for residence times measured in tenths of a second to form the desired products. After cracking, the cracked gas exits the furnace, preferably is quickly quenched, and is sent for further processing downstream. The products formed as a result of the cracking process depend on factors such as the composition of the feed, the hydrocarbon-to-steam ratio, as well as the cracking temperature, pressure and furnace residence time.
Steam cracking in a pyrolysis furnace has been used to successfully produce olefins from light hydrocarbon feedstocks such as ethane, propane, liquefied petroleum gas (LPG), or naphtha. There is, however, a continuing need to reduce the costs associated with the production of olefins. One approach involves the use of lower-cost feedstocks, which can be economically upgraded by pyrolytic cracking processes. For example, the use of feedstocks heavier than the ethane or naphtha feedstocks, which have been the predominant feeds, can possibly reduce the cost of olefin production. Examples of heavy hydrocarbon feedstocks, which may be considered include, but are not limited to, atmospheric gas oil, vacuum gas oil, crude oil, deasphalted oil, oils derived from tar sands or oil shale, gas to liquid heavy ends, heavy condensate and hydrocracked residue. However, the use of these heavier feedstocks having an initial boiling point above 200° C. in a pyrolysis furnace can result in fouling or coking in the convection section tubes, which adversely affects the operation and performance of the furnace. This occurs when the non-vaporized heavy ends are exposed to the high temperatures in the metal tubing within the convection section. It becomes necessary to halt production and clean the pyrolysis furnace at periodic intervals. This significantly reduces output and increases production costs. To reduce this problem, the heavy hydrocarbon feedstock must be completely vaporized in the presence of steam within an unheated surface.
A variety of approaches have been employed in attempting to address the above and other issues associated with the pyrolytic cracking of heavy hydrocarbon feedstock. For example, U.S. Pat. No. 6,632,351 to Ngan, et al. and U.S. Pat. No. 7,311,746 to Stell, et al., which are incorporated by reference as if fully set forth in this specification, each disclose an apparatus for pyrolyzing heavy feedstock, which separates the volatile and non-volatile components of the heavy feed. The light hydrocarbon components, which cause less coking problems, are fed to a steam cracking furnace where they may be converted to olefins. The heavy hydrocarbon components are routed to, for example, a storage tank where they may be processed using other techniques, such as fluid catalytic cracking. However, these approaches require extra processing steps as well as the addition of one or more mixing and separation units prior to the convection section. This significantly increases the process complexity and results in high capital costs.
Another example is provided by U.S. Pat. No. 5,190,634 to Fernandez-Baujin, et al. and U.S. Pat. No. 5,817,226 to Eric Lenglet, which are incorporated by reference as if fully set forth in this specification. Disclosed in these patents are processes in which coke formation is inhibited by using hydrogen-rich gas streams such as H2, CH4 or vaporized light hydrocarbon feedstock to mix and vaporize heavy carbon feedstock. The inclusion of hydrogen products in the convection section minimizes coke formation by, for example, inhibiting polymerization of the preheated hydrocarbons. However, these approaches require additional piping to deliver the requisite hydrogen-rich stream thereby increasing the complexity and cost. The need to use process gases such as H2 also adds to the overall operating costs.
Still another example may be found in U.S. Pat. No. 4,498,629 to Alexander Grondman and U.S. Pat. No. 6,797,238 to Chandrasekharan, et al., which are incorporated by reference as if fully set forth in this specification. These patents disclose an apparatus that minimizes the propensity for coke formation by completely vaporizing the heavy hydrocarbon feedstock with steam prior to being fed to the radiant heating section of the furnace. The apparatus comprises two concentric tubular elements having coinciding longitudinal axes and an annular spaced formed between them. The outer tubular element has a slightly diverging and elongated frusto-conically shaped element attached to its open end at a position, which is behind the location where superheated steam meets the heavy hydrocarbon feedstock. This configuration is stated to reduce the amount of liquid droplets contacting the wall of the frusto-conically shaped element, thereby reducing the risk of coke formation. However, the apparatus requires a very long vaporization length in order to substantially completely vaporize the hydrocarbon feedstock. This adds to the capital cost and requires valuable plot space in the furnace convection section.
Additional methods and systems for cracking heavy hydrocarbon feedstock are disclosed, for example, in U.S. Pat. Nos. 3,617,493; 4,673,486; 5,580,443; 7,090,765; 7,247,765; 7,297,833; 7,312,371; 7,351,872; and 7,358,413 as well as U.S. Patent Appl. Publ. No. 2007/0232845. Each of the aforementioned is incorporated by reference as if fully set forth in this specification. The above prior art approaches disclose various methods and systems that are capable of producing lighter hydrocarbon products from heavy hydrocarbon feedstock with varying degrees of efficiency and of reducing some of the problems associated with coking or fouling. However, there is a continuing need for still further improvements in design and efficiency along with reductions in cost.